Hitherto, various attempts have been made to develop a plant resistant to viral infection (hereinafter, an antiviral plant) by employing recombinant DNA technologies. For example, Powell Abel et al. introduced a cDNA encoding the coat protein of a virus to a plant (Infect Immun., 35, 792-796 (1986)), while the introduction of a defective viral replicase to a plant was attempted by Anderson et al. (P.N.A.S., 89, 8759-8763 (1992)). However, these methods are not practical in that, in order for a plant to have antiviral properties against a number of viruses, all the target viral genes must be introduced to the plant.
To solve this problem, Lodge et al. reported a method for conferring a general antiviral property on a plant by introducing thereto a gene encoding an antiviral protein, e.g., RIP (ribosome inhibiting protein) (P.N.A.S., 90, 7089-7093 (1993)). Although the plant transformed by this method shows an antiviral activity against various viruses, there exists the problem that the antiviral protein may be harmful to human, and hence, the application of the above method may be restricted.
Lactoferrin is a conjugated protein found in mammals, e.g., in human milk, saliva, tear, semen and leukocyte. It affects the growth and differentiation of cells and has an antibacterial and immunopotentiative activity (Arold et al., Science, 197, 263-265 (1977)). Further, it shows an antiviral activity against Friend virus (Lu et al., Cancer Res., 47, 4184-4188 (1987)), human immunodeficiency virus (HIV) and human cytomegalovirus (HCMV) (Meijer et al., J. Disease, 172, 380-388 (1995)).
Mitra and Zhang reported a method for providing a plant having an antibacterial activity by introducing thereto a cDNA encoding human lactoferrin (Plant Physiol., 106, 977-981 (1994)). They confirmed the production of human lactoferrin in a tobacco cell by conducting a western blotting analysis of a tobacco cell transformed with human lactoferrin cDNA and anti-human lactoferrin antibody. They also reported that the proliferation of bacteria was significantly suppressed when the extract of the transformed tobacco cell was added to a culture of bacteria, e.g., Pseudomonas sp., which is pathogenic to tobacco. However, Mitra and Zhang did not, or failed to produce a transformed tobacco plant from the transformed tobacco cells. Moreover, human lactoferrin produced in the transformed tobacco cell was defective, i.e., significant parts thereof were missing, while the antibacterial activity was confirmed only in vitro. Accordingly, the question of whether or not an antiviral plant can be produced from a transformed tobacco cell was left unanswered in the prior art.